instrumentation chat part 3 collimators

two functions of collimator

1. defines field of view

2. rejects unwanted photons

what is collimator made of

• lead

• because of high atomic number and density

septa

• walls of lead between the holes

• absorb off angle photons

• only photons traveling in acceptable directions pass through the holes

spacial resolution

ability to distinguish two nearby radioactive points

sensitivitiy

number of phtons detected per unit of time for a given activity

how are sensitivity and spatial resolution related

inversely

high resolution collimator

• smaller holes

• better resolution

• lower sensitivity

• longer acquisition time to obtain same counts

high sensititvity collimator

• larger holes

• lower resolution

• higher sensitivity

• shorter acquisition time to obtian the same counts

what does smaller hole diameter to

• improve spatial resolution

• decrease sensitivity

what does larger hole diameter do

• decrease spatial resolution

• increase sensitivity

what does longer holes do

• improves spatial resolution

• decreases sensitivity slightly

what does shorter holes do

• decreases spatial resolution

• generally increases sensitivity

what does thicker septa do

• helps prevent prenetration

• decreases open area

septal penetration

• occurs when gamma photons pass through the lead septa instead of bieng absorbed

• happens when photons have enough energy to penetrate the lead

consequences of septal penetration

• poor spatial resolution

• blurring

• star artifacts

• poor image definition

higher energy photons require…

thicker septa to absorb them

example for high energy collimator isotope

I 131

example for medium energy collimator isotope

Ga - 67 and In 111

example for low energy collimator isotope

Xe 133 and Tc 99

what is septal thickness for low energy collimator

thin

what is septal thickness for medium energy collimator

intermediuate

what is septal thickness for high energy collimator

thick

parallel hole collimator

• holes run parallel

• no magnification

• image remains approximately the same size as the object

• most commonly used

converging collimator

• holes angle toward focal point

• produces a magnified image

• used for small organs

• increased geometric efficiency

diverging collimator

• channels diverge outward

• produces smallar/minified image

• increases field of view

• useful for organs larger than detectors normal FOV (kidney)

fan beam collimator

• type of convering collimator

• channels converge to a line parallel to cameras axis of roation

• channels form a wedge like geometry

pinhole collimator

• cone shaped with aperature facing patient

• magnifies the image

• image is inverted

• used for small organs such as the thyroid

why is pinhole collimator useful for imaging small organs

• because they produce magnified images with excellent resolution

characteristics of pinhole colimator

• cone shpaed

• apex faces patient

• thick lead sides

• removable tungsten aperature

• poor sensitivity

• image is reverse

• magnification changes with distance from aperature

parallax errors

• occur in pinhole collimators

• occur because the apparent location of an object changes with the viewing angle

How does increasing the distance between a patient and parallel-hole collimator affect resolution and sensitivity?

• spatial resolution decreases

• image becomes blurrier

• senstivity/count rate changes very slowly with distance

Predict how changing from an all-purpose to a high-resolution collimator affects the time needed to reach the same counts.

imaging time increases

• high resolution has smaller holes so fewer photons reach crystal and decreases sensitivity